The present invention relates to array antennas, and more particularly to an array employing equal amplitude and phase excitations of the radiating elements.
It is well known that array antennas of closely spaced radiating elements will produce a constant gain sector beam on a polar radiation pattern plot, or a flat topped beam on a rectangular radiation pattern plot. In the conventional design, all of the radiators lie in a plane which is essentially perpendicular to the direction of the flat topped beam. The radiating elements must be excited according to values of the function (sin(x))/x where x is in radians. That function changes its magnitude values rapidly, and also undergoes abrupt phase changes of 3.1416 radians. Because of mutual coupling between radiating elements, it is difficult to obtain an array whose elements conform to the desired (sin(x))/x function, especially when the desired sector beam is to cover a large angular region.
Sector beams are used, for example, to give uniform power density over the 3.degree. to 4.degree. sectoral extent of a nation as seen from a geostationary satellite. In terrestrial communication and broadcasting systems it is often desired to uniformly illuminate just one community which may be entirely within a, say, 80.degree. sector as seen from the system's site. Complex power dividers and various lengths of transmission line have been used in the past to achieve the needed sin(x)/x excitations. But, mutual coupling between elements of the array forces a number of trial and error iterations before the desired pattern is obtained. Using the principle of this invention, easy-to-design uniform power dividers and equal length transmission lines to the radiating elements lower the design and fabrication costs. Shaped beams other than constant gain sector beams can be obtained by locating the radiating elements along paths other than the arc of a circle. Where the sector is to be a large angle, such as 120.degree. or more, antennas embodying the invention will work, whereas the conventional sin(x)/x synthesis from a planar aperture will not.